In known plants for the production of sulphur from a gas containing hydrogen sulphide, also known as "acid gas", this gas is caused to flow in a thermal reaction stage in which one third of the hydrogen sulphide is transformed into SO.sub.2 in the presence of oxygen or air at a temperature at least equal to 900.degree. C. The gaseous reaction mixture from the thermal reaction stage contains a certain quantity of sulphur as well as H.sub.2 S and SO.sub.2 in a H.sub.2 S/SO.sub.2 molar ratio of 2/1. This gaseous mixture is subjected to indirect cooling allowing the recovery of calories contained therein, while producing steam, and the cooled mixture is caused to flow to a condensation stage in which the sulphur contained in the gaseous mixture is separated by condensation. In order to improve the sulphur production, which at this stage is still very incomplete, the gaseous mixture issuing from the condensation stage is reheated and then fed into one or more catalytic conversion stages comprising a suitable catalyst, called CLAUS catalyst, at the contact of which SO.sub.2 reacts with H.sub. 2 S in order to form a fresh quantity of sulphur. The residual gases issuing from the final catalytic conversion stage are caused to flow to an incineration stage, the effluent of which are thereafter discarded in the atmosphere.
The use of a thermal reaction stage to oxidize into SO.sub.2 the required quantity of H.sub.2 S is only possible for H.sub.2 S concentrations of acid gas higher than about 15 to 20% by volume. For values lower than that limit, it is not possible to maintain a sufficient flame temperature, i.e. in the range of 900.degree. C. or more, to obtain a stable combustion without necessitating important modifications.
Furthermore, this high-temperature thermal reaction leads to a transformation of part of the sulphur produced into COS and CS.sub.2 when the acid gas contains CO.sub.2 and/or hydrocarbons. The formation of these organic sulphur compounds is bothersome in so far that they can only be transformed with difficulty into sulphur in the catalytic conversion stages, and this contributes consequently to the decrease of the overall sulphur conversion yield of a sulphur plant.
In order to overcome these drawbacks, resulting from the use of a thermal reaction stage for producing effluent containing H.sub.2 S and SO.sub.2 in a H.sub.2 S/SO.sub.2 ratio of 2/1, German patent application No. 3,015,800 proposes replacing the thermal reaction stage of a sulphur plate with a catalytic oxidation stage using an oxidation catalyst based on a vanadium oxide or sulphide supported on a non-alkaline porous refractory matrix and operating at temperatures lower than 454.degree. C., the said temperatures being controlled by recycling part of the gaseous effluent from the catalytic oxidation stage, after cooling of said effluent and condensation of the sulphur contained therein.
In such a process, the necessity of recycling part of the effluent of the catalytic oxidation stage for controlling the temperature of said stage causes a certain number of technological restrictions and leads especially to the use of quantities of catalyst in the oxidation stage, which are more important as the H.sub.2 S concentration in the acid gas is high.
In French patent application No. 81 15900 filed on Aug. 19, 1981, description is made of a catalytic process for the production of sulphur from a gas containing H.sub.2 S, which is similar to the process disclosed in the German patent application referred to above but for the oxidation catalyst used which consists of a metal compound, more particularly an iron compound, associated to a porous carrier comprising a major proportion of an oxide selected from silica, titanium oxide or zirconium oxide. The use of such an oxidation catalyst allows operating at high temperatures, that can reach in particular 700.degree. C., thus preventing the recycling operation of a part of the effluent from the oxidation stage, after its cooling, destined to ensure the temperature control in the oxidation stage, and thus eliminating the restrictions of such a recycling.
Said process suffers however from the drawback that the oxidation catalyst cannot be used at temperatures higher than 700.degree. C., that is to say at temperatures which would be closer to the temperatures selected for the thermal oxidation stage in the known sulphur plants and accordingly would lead to increasing the sulphur yield of the catalytic oxidation stage.